Dual band photodiode element and method of making the same

ABSTRACT

Mercury cadmium telluride (MCT) dual band photodiode elements are described that include an n-type barrier region interposed between first and second p-type regions. The first p-type region is arranged to absorb different IR wavelengths to the second p-type region in order that the photodiode element can sense two IR bands. A portion of the second p-type region is type converted using ion-beam milling to produce a n-type region that interfaces with the second p-type region and the n-type barrier region.

This application is a divisional of U.S. application Ser. No. 16/960,416filed on Jul. 7, 2020, which claims priority to UK Patent Application1800275.8 filed in the United Kingdom on Jan. 8, 2018. The entirecontent of each prior application is hereby incorporated by reference.

The invention relates to a photodiode element responsive to dual bandelectromagnetic radiation, e.g. infrared radiation, a method of makingsaid photodiode element and detector arrays comprising multiple of saidphotodiode elements.

Dual waveband infrared focal plane arrays offer several advantages overtheir single waveband counterparts, including enhanced contrast forsignal processing and scene interpretation, and the ability to provideabsolute object temperature measurement.

Mercury cadmium telluride (MCT) is a common material used forfabricating dual waveband infrared detectors. The cadmium mole fractionin a particular layer, termed x, determines the cut-off wavelength ofthat layer, with higher x corresponding to shorter wavelengths. Forexample, x is typically 0.22 and 0.27 for longwave infrared (LWIR) andmedium wave infrared (MWIR) absorbers with 10 and 6 μm cut-offwavelengths respectively. The principle can be applied to any materialssystem where the band gap can be engineered to provide the desiredcut-offs.

FIG. 1 illustrates a portion of a mesa array 1 of dual-band infraredphotodiode elements 2, each comprising back to back p-n photodiodejunctions 3,4. Each element 2 includes a first n-type absorber layer 6arranged to absorb medium wave infrared (MWIR) and a second n-typeabsorber layer 7 arranged to absorb longwave infrared (LWIR). The twop-n photodiode junctions 3,4 are separated by a wide band gap p-typebarrier layer 8. The aforementioned layers 6, 7, 8 are grownmonolithically from MCT onto a transparent common layer 9 also of MCT bya technique such as MBE or MOCVD.

An indium bump 10 is provided on the face of each mesa for electricalconnection to a readout integrated circuit 11. Infrared light enters thearray from the substrate 9 side, the shorter wavelength MWIR beingabsorbed by the first n-type layer 6 and the longer wavelength LWIRbeing absorbed by the second n-type layer 7. The first n-type layer 6acts as a filter preventing MWIR light reaching the second n-type layer7.

When a voltage is applied across the device one of the PN junctions 3,4will be in reverse bias and the other in forward bias; photocurrent fromthe n-type absorber layer 6,7 that is directly adjacent to the reversedbiased PN junction 3,4 will appear in the external circuit. Thus asignal corresponding to LWIR absorption will be obtained with one biaspolarity and a signal corresponding to MWIR absorption with the oppositebias polarity. During operation, the bias across the back-to-backphotodiode junctions 3,4 can be sequentially switched to provide LWIRand MWIR signals that are spatially but not temporally coherent.

FIG. 2A illustrates a known design of mesa detector element 20 thatprovides temporal coherence. The element 20 comprises an n-type barrierlayer 21 interposed between a shorter wave (SW) n-type absorber layer 22and a longer wave (LW) n-type absorber layer 23. A p-type window layer24, transparent to both SW and LW is arranged adjacent the SW absorberlayer 22 to form a first PN-junction 25, and a p-type cap layer 26 isprovided over the LW absorber layer 23 to form a second PN-junction 27;the first and second PN junctions 25,27 are arranged back-to-back.

The top of the mesa element 20 is provided with two electrical bumpcontacts 28,29 for connection by bump bonding to a readout circuit (notshown) to enable both photodiodes to be simultaneously reverse-biased. Afirst of the electrical contacts 28 is arranged for ohmic contact withthe p-type cap 26, while a second electrical contact 29 connects to anoverlay contact 31 which shorts the p-cap layer 26 such that the secondcontact has a direct ohmic connection to the n-type LW absorber 23. Toachieve this an etched slot 30 is provided through the p-type cap 26,between the two contacts 28, 29. The slot 30 acts to electricallyisolate the first and second contacts 28,29. Ensuring the first andsecond contacts 28,29 are electrically isolated from each other andavoiding shorting of the photodiode layers, requires careful control ofthe etching, passivation and metallisation procedures.

The first and second PN-junctions 25,27 are simultaneously reversedbiased by applying a voltage across the element 20 such that the secondcontact 29 is positive and the first contact 28 and the common contact32 are negative. SW light absorbed by the n-type SW layer 22 generates aphotocurrent between the second contact 29 and common contact 32 withelectron flow through the n-type barrier 21 and LW absorber 23 to thesecond contact 29 via the overlay contact 31. LW light absorbed at then-type LW absorber 23 generates a photocurrent between the secondcontact 29 and first contact 28 with corresponding electron flow throughthe n-type LW absorber 23 to the second contact 29. As such, the signalat the second contact 29 is representative of the combined absorption ofLW and SW, whilst the signal at the first contact 28 is representativeof the absorption of LW. The strength of the SW absorption can bedetermined by subtracting the LW signal from the combined signal.

FIG. 2B is variant design of element 30 operating in a similar manner tothat of FIG. 2A but having a NPN rather than PNP configuration. Thefirst contact 31 is in direct electrical contact with the top n-type LWabsorber layer 32. An edge of the mesa element 30 is etched down to theintermediate p-type barrier layer 33 and an overlay 34 provided to placethe p-type barrier layer 33 in ohmic connection with the second contact35. An insulating layer 36 isolates the second contact 35 from the topn-type LW absorber 32. In operation the polarities of the first contact31, second contact 35 and common (not shown) are reversed compared withthat of the previous example in order to simultaneously reverse bias theback-to-back diodes.

The need to provide a contact to an intermediate layer complicates themanufacture of the elements 20, 30 of FIGS. 2A and 2B. Further, theremoval of material from the top of the mesa element 30 of FIG. 2B,results in a blind spot to LW. Additionally, the removal of materialincreases the minimum pitch of the element achievable.

FIG. 3 illustrates a further known design of detector element 40. Theelement 40 comprises a p-type barrier layer 41 that is interposedbetween a p-type SW absorber 42 and p-type LW absorber 43. The laminateis formed on a substrate layer 44 of CdZnTe on which is provided ann-type window common layer 45. The n-type window common layer 45 withp-type SW absorber 42 provide the SW photodiode 46. The LW photodiode 47is formed by doping (ion implantation) the p-type LW absorber 43 toprovide an n-type surface region 48 within the p-type layer 43. A firstcontact 49 is located on the n-type region 48 and a second contact 50 islocated directly on the p-type LW absorber 43. A voltage is placedacross the element 40 such that the common contact (not shown) and firstcontact 49 have a positive polarity and the second contact 50 has anegative polarity. The signal at the second contact 50 is proportionalto combined SW and LW adsorption and the signal at the first contact 49to LW absorption.

This design avoids the blind spot problem of the arrangements of FIG.2B. The implanted n-type region needs to be larger than the electricalcontact to provide tolerance in the positioning of the first contact 49.This limits the minimum pitch achievable for an array of elements 40.Additionally, the use of ion implantation requires the additionalsubsequent step of a high temperature activation anneal necessitating aseparate photolithography stage for adding the contact.

The present invention was conceived to ameliorate the problems of theprior art.

According to a first aspect of the invention there is provided aphotodiode element responsive to dual band radiation; the photodiodeelement comprising: a first p-type region; a second p-type region; afirst n-type region arranged between the first p-type region and thesecond p-type region to provide PN-junctions; one or more of the firstp-type region, second p-type region and first n-type region providing afirst absorbing region for absorbing a shorter wave band and a secondabsorbing region for absorbing a longer waveband; the first absorbingregion and the second absorbing region being arranged on opposing sidesof an n-type barrier provided by the n-type region, the n-type barrierbeing substantially non-absorbent to the longer wave band; thephotodiode further comprising a second n-type region that interfacessuch as to be in electrical and physical contact with the first n-typeregion; and two ohmic electrical contacts, a first arranged on andcontacting the second p-type region and a second on and contacting thesecond n-type region.

Through this arrangement electron flow between the second contact andthe first n-type region can occur via the second n-type region,precluding the need for an etched slot. The arrangement can also bemanufactured using process steps that are only a small variation fromthe more straightforward method used to manufacture the sequential modedesign of FIG. 1 making it more straightforward to produce than thedesign of FIG. 3 .

Through the afore described arrangement, there will exist an interfacebetween the second p-type region and the second n-type region providinga portion of the PN-j unction associated with absorption of the longerwaveband. A plane in which a portion of the PN-junction provided betweenthe second p-type region and the second n-type region is non-parallel toand intersects another portion of the PN-j unction provided between thesecond p-type region and first n-type region. The portion of thePN-junction provided in part by the second n-type region may extend to asurface of the photodiode element on which the first and second contactslie.

The second n-type region may be a portion of the second p-type regionthat has been type converted. Type conversion may be by, for example,ion milling. As such the second n-type region may comprise a recessformed by ion-milling. Advantageously the higher x of the middle n-layerbarrier self-limits the type conversion to the second p-type region,i.e. such that the first p-type region will not be converted.

Because the ion-milling type conversion process can be carried out atroom temperature, a single photolithography stage can be used to bothdefine the type converted second n-type region and metalize to providethe second contact on the second n-type region. As the ion millingprocess means the second n-region is self-aligned to the second contactthe second n-region can advantageously be of smaller area compared ton-type region type formed through type conversation by implantation,where some allowance has to be given for process tolerances in aligningthe contact. This allows potentially smaller element pitches to beachieved.

Where the photodiode element is of mesa form, a further advantage ofusing an ion-milling process is that the conversion process can becarried out using the same ion beam etcher as used to delineate the mesaslots.

The first p-type region may provide, at least in part, the firstabsorbing region, and the second p-type region may provide, at least inpart, the second absorbing region.

In one arrangement the first p-type region may wholly provide the firstabsorbing region and the second p-type region plus second n-region maywholly provide the second absorbing region.

In such an example the second n-type region and the second p-type regionmay interface, i.e. be in direct physical and electrical contact withthe n-type barrier.

Alternatively the first absorbing region and/or the second absorbingregion may be provided, at least in part by the first n-type region. Assuch the n-type region may also comprise an n-type absorption region forabsorbing the shorter band and/or an n-type region for absorbing thelonger band. The n-type region for absorbing the shorter band may beprovided between the n-type barrier and the first p-type layer. Then-type region for absorbing the longer band may be provided between then-type barrier and the second p-type layer.

In one arrangement the n-type region may define one or both of the firstand second absorbing regions alone. Where the n-type region provides thesecond absorbing region alone, the second p-type region and secondn-type region may act as a cap.

In another arrangement, the first absorbing region may be provided by acombination of the first p-type region and the first n-type region andthe second absorbing region provided by a combination of the secondp-type region and the first n-type region.

The first n-type region barrier layer may be substantially non-absorbentto the shorter wavelength radiation

The photodiode element may in addition comprise a further p-type layerthat is relatively highly conductivity compared with first and secondp-type regions. The further p-type layer may interface the first p-typeregion. The further p-type later may be common to multiple photodiodeelements. The further p-type layer may be substantially non-absorbent toeither the longer or shorter wave band.

The first and second ohimic electrical contacts may be provided bymetallic material.

The photodiode element may be connected to a third ohimic electricalcontact (e.g. provided by metallic material). The third ohimicelectrical contact may be arranged on the other side of the n-typebarrier to the first and second contacts. The third ohimic electricalcontact may be in direct contact with the further p-type layer.

The photodiode element may be comprised from MCT. The photodiode elementmay have a mesa structure.

The first and second contacts may provide contact between the elementand a read out integrated circuit (ROIC). The first and second contactsmay be bump contacts (e.g. comprise indium) for bump bonding the elementto the ROIC. The second contact may be formed over the recess within thesecond n-type region.

The shorter wave band and longer wave band are typically discrete, i.e.not immediately adjacent one another. The photodiode element may beoperative to any two bands of infrared radiation between SWIR and VLWIR.In one arrangement a first band may lie partially or wholly in MWIR anda second band may lie partially or wholly in LWIR.

In another aspect of the invention there is provided a detector arraycomprising photodiodes elements as variously described above. Thedetector array may comprise a focal plane array comprising thephotodiode elements that is bonded to a ROIC.

According to a second aspect of the invention there is provided a methodof manufacturing a photodiode element, the method comprising:

form a first p-type layer;

form a first n-type region that includes a n-type barrier layer on thefirst p-type layer;

form a second p-type layer on the first n-type region; and

type convert a portion of the second p-type layer to provide a secondn-type region that is in electrical and physical contact with both thesecond p-type layer and the first n-type region;

provide a first electrical contact on the second p-type layer and asecond electrical contact on the second n-type region.

The portion of the second p-type region may be type converted by ionbeam milling to provide the second n-type region. The second p-typeregion may be comprised from a MCT semiconductor.

The method may comprise forming a passivation layer over the element anda window in the passivation layer through which the second p-type layeris exposed. The method may further comprise ion-milling through thewindow to convert the exposed portion of the second p-type layer to thesecond n-type region.

The method may include depositing a metal through the window to form acontact.

The method may include forming a mask over the element that leaves thewindow exposed, ion beam milling through the mask to convert the exposedportion of the second p-type layer to the second n-type region, anddepositing a metal through the mask to form a contact with the n-typeregion.

Before forming a passivation layer, the method may include providing asecond mask over the element, forming the passivation layer over theelement, and removing the second mask to provide the window in thepassivation layer to expose a portion of the second p-type layer.

The method may include etching through the second p-type layer, firstn-type region and first p-type layer to form a mesa photodiode element.

The method may further including growing a p-type common layer on asubstrate and growing the first p-type layer on the p-type common layer.

The invention will now be described by way of example with reference tothe following figures in which:

FIG. 1 is a schematic side view of part of an array of photodiodeelements of the prior art;

FIG. 2A is a schematic side view of second example of a prior artphotodiode element that provides spatially and temporally coherent dualband signals;

FIG. 2B is a schematic side view of third example of prior artphotodiode element that provides spatially and temporally coherent dualband signals;

FIG. 3 is a schematic side view of further example of prior artphotodiode element that provides spatially and temporally coherent dualband signals;

FIG. 4 is a schematic of a portion of an infrared focal plane array ofphotodiode elements for providing spatially and temporally coherent dualband signals.

FIG. 5 is a schematic side view of one of the photodiode elements ofFIG. 4 ;

FIG. 6 is a schematic side view of a variant photodiode element forproviding spatially and temporally coherent dual band signals; and

FIG. 7 is a schematic side view of a further variant photodiode elementfor providing spatially and temporally coherent dual band signals.

FIG. 4 illustrates mesa photodiode elements 101 of a focal plane array100 for providing spatial and temporal coherent dual band signals forreceipt by a readout integrated circuit (ROIC) 200. Note that FIGS. 4-7do not necessarily show the layers and regions to scale. The arrowsrepresent the direction of electron flow of the photocurrents when thephotodiodes are reversed biased.

With reference to FIGS. 4 and 5 , the element 101 comprises a relativelyhigh conductivity p-type layer 103 that is common to the elements 101 ofthe array 100. Provided on the high conductivity p-type layer 103 is afirst p-type layer 104 for absorbing a wave band of a shorterwavelengths (shorter wave band) of IR (e.g. lying within MWIR), a n-typebarrier layer 105 formed on the first p-type layer 104, and a secondp-type layer 106 for absorbing a wave band of longer wavelengths (longerwave band) of IR (e.g. lying within LWIR) formed on the n-type barrierlayer 105 such that the n-type barrier layer 105 is interposed betweenthe first and second p-type layers 104,106. The n-type barrier layer 105is substantially transparent to both the shorter wave band and thelonger waveband of IR.

The element 101 further includes an n-type region 107 that extends froma top 101A of the mesa element 101 to interface with the n-type barrier105 such that they are in physical and electrical contact. The n-typeregion 107 is provided by type-converting a portion of the second p-typelayer 106 using ion beam milling. This technique is described inWO2011/067058 Jones & Bains published 9 Jun. 2011 hereby incorporated byreference in its entirety. The n-type region 107, like the second p-typeregion 106 absorbs the longer wave band IR. As result of the ion beammilling process 107 the n-type region has a recess 108 that extends fromthe top of the mesa 101A towards the n-type barrier 105.

The element 101 described comprises two PN-junctions, a first 109extends parallel to the top 101A of the mesa element 101, formed betweenthe first p-type layer 104 and the n-type barrier 105, and a second 110provided in part between the second p-type layer 106 and the n-typebarrier 105 and in part between the second p-type layer 106 and then-type region 107. A first portion of the second PN junction 110provided between the second p-type layer 106 and n-type barrier 105 liessubstantially parallel to the mesa top 101A, whereas a second portion ofthe second PN-junction 110 between the second p-type layer 106 and then-type region 107 extends laterally away from a plane in which the firstportion of the PN junction 110 lies, to the top surface 101A of the mesaelement 101.

The element 101 further includes a first metallic electrical contact 111and a second metallic electrical contact 112. The first and secondcontacts 111, 112 are provided by respective first and second indiumbumps on the top 101A of the mesa element 101. The first contact 111 isprovided on and in ohmic contact with the second p-type layer 106. Thesecond contact 112 is on and in ohmic contact with the n-type region107. Each element 101 is connected to a third electrical contact 113(see FIG. 4 ), via the common high conductive p-type layer 103. Thethird contact 113 is provided by an indium bump on the high conductivep-type layer 103. Nevertheless, in principle, each element 101 may beprovided with its own third contact.

The n-type region 107 provides electrical connection between the secondcontact 112 and the n-layer barrier 105 whilst electrically isolatingthe second contact 112 from the first contact 111 on second p-type layer106.

In operation, both PN junctions 109,110 are reverse biasedsimultaneously by applying a voltage across the element 101 such thatthe first contact 111 and common 113 are negative and the second contact112 is positive. Absorption of the shorter wave length light in thefirst p-type layer 104 proximate the first PN junction 109 results inelectron flow (indicated by vertical arrow on FIGS. 4 & 5 ) through then-type barrier 105 to the second contact 112 via the n-type region 107,i.e. a photocurrent between the second contact 112 and common 113.

Longer wavelength light absorbed by the second p-type layer and secondn-type region proximate the second PN-j unction 110 produces an electronflow (represented by horizontal arrow of FIGS. 4 & 5 ) through thesecond p-type layer 106 and n-type region 107 towards the second contact112, i.e. a photocurrent between the second contact 112 and firstcontact 111. Note the n-type barrier 105 acts as a barrier to hole flowsubstantially preventing the flow of holes generated by absorption ofthe shorter waveband to the first contact 111 and similarly holesgenerated through absorption of the longer wave light in the secondp-type layer 106 to the common 113.

FIG. 6 illustrates a variant design of element 101′ in which like partshave are given the same numbers as FIG. 5 . The element 101′ furthercomprises a shorter wave n-type absorbing layer 105A on one side of thebarrier 105 directly adjacent the first p-type absorber 104 so as toprovide the first PN junction 109′, and a longer wave n-type absorberlayer 105B arranged on the other side of the barrier 105 directlyadjacent the second p-type layer 106 to provide a portion of second PNjunction 110′. The longer wave n-type absorber layer 105B lies in directphysical and electrical contact with the n-type region 107. In thisarrangement electron-holes pair can be generated by absorption of MWIRin either of the first p-type layer 104 or shorter wave n-type absorber105A to provide a MWIR signal. Similarly electron-holes pairs can begenerated by absorption of LWIR in either the longer wave n-typeabsorber layer 105B, the second p-type layer 106 or n-type region 107 toprovide the LWIR signal.

FIG. 7 illustrates a variant design of element 101″ to that of FIG. 6 inwhich the second p-type layer 106′ is a wide band gap cap provided by ahigh x material which is transparent to both shorter and longerwavebands. The p-type cap 106′ is type converted using ion-milling toprovide an n-type cap 107′.

The n-type cap 107′ provides a conduit for electron flow between then-type barrier 105 and second contact 112, and the longer wave n-typeabsorber 105B and the second contact 112.

The arrangement of FIG. 7 offers the potential for lower dark currentbecause the higher bandgap of the p-type cap 106′ compared with longerwave n-type absorber 105B provides that the PN—junction 110′ is aheterojunction rather than a homojunction.

The above examples can all be grown as a monolith from MCT usingepitaxy, MOCVD or the like using techniques known to those skilled inthe art.

The following provides an example procedure for the manufacture of anarray of photodiode elements of FIG. 4 :

a) Epitaxially grow MCT wafer on a suitable substrate such as GaAs; thecomposition of the MCT is controlled during growth to provide a stackedlayered comprising the first p-type layer 104, n-type barrier 105 andsecond p-type layer 106 so as to provide first and second PN junction109, 110. Arsenic is included with the MCT during growth to form thep-type layers and iodine to form the n-type layer. In one variant p-typeextrinsic impurities may be introduced during the growth of the MCTlayers. The different band gaps of the layers 104, 105, 106 are achievedthrough using different cadmium mole fractions (x) as is known to thoseskilled in the art.b) Define a mesa array pattern by photolithography and etch to form mesaslots using a combination of dry and wet etching so as to provide arrayof mesa elements 101.c) Define and apply a resist pattern comprising first and second padseach on top surface 101A of the mesa elements 101 usingphotolithography.d) Deposit a passivation layer (e.g. of CdTe or CZT) and lift off theresist pattern (first and second pads) to define first and secondwindows in the passivation layer on the top of each mesa through whichthe second p-type layer 106 is exposed.e) Anneal the wafer to interdiffuse the passivation layer with the MCTmaterial.f) Define and deposit a second resist pattern that covers the mesaelement except the first windows.g) Using a suitable pre-treatment, deposit an ohmic p-contact barriermetal.h) Lift-off the resist to leave the metallisation in the p-contactwindow.i) Define and apply a third resist pattern that covers the mesa elementsexcept the second windows.j) Ion-beam mill through the second mask and second window to form then-type region such that it extends down to the n-type barrier layer 105.k) Deposit an ohmic n-contact barrier metal through the second mask tocontact with the type converted n-type region.l) Remove the third resist.m) Define and apply a fourth resist pattern that covers the mesa exceptthe p-contact barrier metal and n-type barrier metal.n) Deposit an indium bump metallisation layer.o) Lift-off the fourth resist to provide indium bump contacts.p) Dice and bump-bond onto a suitable ROIC.

The structure of FIG. 6 is produced through variation of the cadmiummole fractions (x) during formation of the sandwiched n-type layer toprovide layer 105A and 105B. The p-type cap layer is formed by using arelatively large cadmium mole fraction (x).

The common contact windows can be provided by a number of un-passivatedmesas, normally but not exclusively located near the edge of the array.A portion of the p-type common layer can be metallised during step g toprovide the common contact.

In a variant to step a), rather than introducing extrinsic impurities,vacancies may be introduced to create the p-type layer by a post-growthanneal.

Rather than forming first and second windows by masking at step c, oneor both may be formed by wet and/or dry etching the passivation layer.

Although the above examples are described using ion beam milling to typeconvert the second p-type layer to the n-type region, conversion mayinstead be achieved using impurity in-diffusion, e.g. of Hg, and anodicoxidation.

The photodiode element and detector array comprised therefrom may beconfigured to be operative at wavebands other than MWIR and LWIR. Morebroadly the photodiode element structure could be applied to detectorscomprised from materials other than MCT in order to providesensitivities to wavebands other than IR.

The invention claimed is:
 1. A photodiode element that is responsive todual band radiation, the photodiode element comprising: a first p-typeregion; a second p-type region; a first n-type region arranged betweenthe first p-type region and the second p-type region to providePN-junctions; one or more of the first p-type region, second p-typeregion and first n-type region being configured to provide a firstabsorbing region for absorbing a shorter wave band relative to a longerwaveband, and a second absorbing region for absorbing the longerwaveband, the first absorbing region and the second absorbing regionbeing arranged on opposing sides of an n-type barrier provided by then-type region, the n-type barrier being substantially non-absorbent tothe longer wave band; the photodiode comprising: a second n-type regionthat interfaces with the first n-type region; and two ohmic electricalcontacts, a first arranged on and contacting the second p-type regionand a second arranged on and contacting the second n-type region.
 2. Aphotodiode element according to claim 1, wherein the second n-typeregion interfaces with both the second p-type region and the firstn-type region.
 3. A photodiode element according to claim 1, wherein thefirst n-type region interfaces with at least a portion of the secondp-type region that has been type converted.
 4. A photodiode elementaccording to claim 1, wherein the first p-type region provides, at leastin part, the first absorbing region and the second p-type regionprovides, at least in part, the second absorbing region.
 5. A photodiodeelement according to claim 1, wherein the second n-type region and thesecond p-type region interfaces with the n-type barrier.
 6. A photodiodeelement according claim 5, wherein the first n-type region consists ofthe n-type barrier layer.
 7. A photodiode element according to claim 1,wherein the first absorbing region and second absorbing region areprovided, at least in part, by the first n-type region.
 8. A photodiodeelement according to claim 7, wherein the first absorbing region isprovided by the first p-type layer and the first n-type region, and thesecond absorbing region is provided by the second p-type region and thefirst n-type region.
 9. A photodiode element according to claim 1,wherein one or more of the first p-type region; second p-type regionfirst n-type region and second n-type region are formed from amercury-cadmium-telluride semiconductor.
 10. A photodiode elementaccording to claim 1, being configured with a mesa photodiode elementform.
 11. A detector array comprising: a combination of multiplephotodiode elements, each configured according to claim 1.